Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (“CDMA”) systems, time division multiple access (“TDMA”) systems, frequency division multiple access (“FDMA”) systems, 3GPP Long Term Evolution (“LTE”) systems, and orthogonal frequency division multiple access (“OFDMA”) systems.
Generally, a wireless multiple-access communication system can simultaneously support communications for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communications link from the base stations to the terminals, and the reverse link (or uplink) refers to the communications link from the terminals to the base stations. This communications link may be established via a single-in-single-out, multiple-in-signal-out, or a multiple-in-multiple-out (“MIMO”) system.
A MIMO system employs multiple (“NT”) transmit antennas and multiple (“NR”) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system supports both time division duplex (“TDD”) and frequency division duplex (“FDD”) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point to extract transmit beamforming gain on the forward link when multiple antennas are available at the access point.
Generally, wireless cellular communication networks incorporate a number of mobile user equipments (“UEs”) and a number of base nodes (“NodeBs”). A NodeB is generally a fixed station, and may also be called a base transceiver system (“BTS”), an access point (“AP”), a base station (“BS”), or some other equivalent terminology. As improvements to networks are made, the NodeB functionality has evolved, so a NodeB is sometimes also referred to as an evolved NodeB (“eNB”). In general, NodeB hardware, when deployed, is fixed and stationary, while UE hardware is portable.
In contrast to a NodeB, a mobile UE can comprise portable hardware. A UE, also commonly referred to as a terminal or a mobile station, may be a fixed or mobile device, and may be a wireless device, a cellular phone, a personal digital assistant (“PDA”), a wireless modem card, and so on. Uplink communication (“UL”) refers to a communication from a mobile UE to a NodeB, whereas downlink (“DL”) refers to a communication from a NodeB to a mobile UE.
Each NodeB contains radio frequency transmitter(s) and the receiver(s) used to communicate directly with the mobile UEs, which move freely around it. Similarly, each mobile UE contains radio frequency transmitter(s) and receiver(s) used to communicate directly with a NodeB. In cellular networks, the mobile UEs cannot communicate directly with each other but have to communicate with the NodeB.
A reference signal (“RS”) is a pre-defined signal, pre-known to both transmitter and receiver. The RS can generally be thought of as deterministic from the perspective of both transmitter and receiver. The RS is typically transmitted in order for the receiver to estimate the signal propagation medium. This process is also known as “channel estimation.” Thus, an RS can be transmitted to facilitate channel estimation. Upon deriving channel estimates, these estimates are used for demodulation of transmitted information. This type of RS is sometimes referred to as Demodulation RS or DM-RS. Note that a RS can also be transmitted for other purposes, such as channel sounding (sounding reference signal or “SRS”), synchronization, or any other purpose. Also note that a RS can sometimes be called a pilot signal, a training signal, or any other equivalent term.
In many modern communication systems, the DM-RS may be specific to a UE (herein referred to as a “UE-RS”, or User-Equipment Specific Reference Signal). For example, Releases 9 and 10 of LTE rely on UE-RSs for demodulation. Different spatial processing techniques are considered in newer releases of LTE, such as single-user MIMO (“SU-MIMO”) with up to eight layers of transmission, and multi-user MIMO (“MU-MIMO”) operation that is cooperative and coordinated across multiple cells and nodes in the network. The cooperation can be at the level of beam coordination where the transmission point for each user is still in one cell, or it can be at the level of joint transmission where the data packet for a particular UE is transmitted from multiple cells. UE-RSs are instrumental in enabling efficient operation under these different spatial processing techniques.
Further, UE-RSs provide UEs with a local channel estimate among the resources assigned to the UE and are treated as being transmitted using a distinct antenna port with its own channel response. A typical usage of a UE-RS is to enable beamforming of the data transmissions to specific UEs. For example, rather than using the physical antennas used for transmission of the other (cell-specific) antenna ports, the eNodeB may use a correlated array of physical antenna elements to generate a narrow beam in the direction of a particular UE. Such a beam may experience a different channel response between the eNodeB and the UE, thus requiring the use of UE-specific RSs to enable the UE to demodulate the beamformed data coherently.
In order for the UE to be able to carry out the demodulation and decoding of the transmitted data, UE-RS information needs to be conveyed to the UE. Such information enables the UE with knowing the resource elements (“REs”) used for data symbols, performing channel and interference estimation, among other procedures. What is needed, therefore, is a way to efficiently transmit UE-RS information to a UE. The present application addresses this issue.